1. Field of the Invention
The present invention relates to a lighting device, a projector including the lighting device, and a lighting method.
2. Description of the Related Art
In recent years, large-screen displays have become widespread. Large-screen displays are commonly used in conferences, presentations, training seminars, or the like. Such large-screen displays include various types of displays such as a liquid crystal display or a plasma display, and an appropriate display is selected among these in accordance with a range of a location or the number of participants. A display which can project an image on a projection surface such as a screen so as to display an enlarged image, namely, a projector, is selected among the various types of displays. This projector is the most widespread large-screen display because it is relatively inexpensive, and has a good portability (namely, a compact, lightweight, and portable projector).
Within such a background, situations which require communication are increasing nowadays. For example, many small conference rooms or partitioned meeting spaces are provided in an office, and conferences or meetings using a projector are often conducted.
As a lighting device for use in such a projector, a projector using a high brightness discharge lamp (for example, extra-high-pressure mercury lamp) as a light source is known. This discharge lamp requires time until stable emission after the start of lighting while achieving high brightness at low cost. For this reason, as a substitute light source for the discharge lamp, a light-emitting element such as red (R), green (G), and blue (B) light-emitting diodes (LED) or an organic EL element has been proposed, and is put into a practical use.
As a lighting device using such a light-emitting element, a lighting device is known which irradiates a fluorescent body with excitation light from an excitation light source (first light source) and generates red (R), green (G), and yellow (Y) light from the fluorescent body (refer to Patent Document 1, JP 2011-191602A). The lighting device described in Patent Document 1 uses a blue laser diode or the like as the excitation light source (first light source), and irradiates the fluorescent body with the blue excitation light from the excitation light source, so as to excite the fluorescent body, so that red (R), green (G), and yellow (Y) fluorescence is obtained. The light of red (R) and green (G) wavelength ranges is obtained by a color wheel on a time-division basis from the fluorescence of red (R), green (G), and yellow (Y) wavelength ranges, and emit from an emission optical path.
In the lighting device described in Patent Document 1, for example, a blue laser diode light source (second light source), which is provided separately from the excitation light source, emits blue (B) light, and this blue light joins the emission optical path for red and blue light after passing through the color wheel by an optical member (color composite prism). The lighting device sequentially emits the red, green, and blue light on a time-division basis. In the projector using such a lighting device, the red, green, and blue light sequentially emitted from the lighting device is directed to a light modulator, and the light is gradation-controlled with respect to each pixel by the light modulator, so that a color projection image is formed.
In the above-described lighting device, the red and green light passes through the color wheel, so that the lighting device can emit the red and green light of a desired tone and color purity. However, the blue light from the second light source directly joins the emission optical path without passing through the color wheel. For this reason, the blue light of a desired tone and color purity cannot be obtained. The color purity of the blue light cannot be improved. Thus, the reproduction of an accurate color image is limited.